The invention pertains to improvements to equipment for testing microcircuits. This is important because the manufacturing processes for microcircuits cannot guarantee that every microcircuit is free of defects. Dimensions of the contacts or leads for the individual microcircuits are microscopic, on the order of tenths of a mm., and process steps very numerous and complex, so small or subtle failures in the manufacturing process can often result in defective devices.
Usually (but not always) a microcircuit is mounted within a small plastic housing or package that protects the microcircuit within from damage. The individual microcircuit contacts are connected within the package to leads external to the package. The pitch, or center-to-center spacing of adjacent leads, may be as small as 0.4 mm. for some packages.
A number of different designs exist for the housing or package of the microcircuit. Some have cantilevered leads projecting from the bottom edge of the package. Others have small contacts that are flush with or project only slightly from the package surface. The leads of others are small solder balls that are melted during installation on a circuit board. In any case, the usual practice is to solder each of the many leads of a package to a larger circuit board that provides for connections to switches and other discrete components, transducers such as speakers, etc. A circuit board often interconnects a number of microcircuits as well.
Mounting a defective microcircuit on a circuit board is relatively costly. Once soldered to a circuit board, removing a microcircuit is problematic because the very act of melting the solder for a second time may ruin the circuit board. Thus, if the microcircuit is defective, the circuit board itself is probably ruined as well, meaning that the entire present value of the circuit board is lost. Even if it is possible to remove a defective microcircuit, detecting which of perhaps several is defective is difficult and expensive. For all these reasons, a microcircuit is usually tested before installation on a circuit board.
Each microcircuit must be tested in a way that identifies a very high percentage of the defective microcircuits, but yet does not improperly identify more than a small percentage of good microcircuits as defective. Failure to identify defective microcircuits in particular adds substantial overall cost to the final product. Wrongly identifying good microcircuits as defective if frequent, can also add significant costs to microcircuit production.
The need to accurately test microcircuits has led to the development of dedicated equipment for testing microcircuits. Reliable test equipment faces a number of challenges that makes the test equipment itself quite complex. First of all, the test equipment must make accurate, low resistance, temporary electrical contact with each of the closely spaced microcircuit leads without damaging either the leads or the microcircuit. Because of the small size of microcircuit leads and the spacing between the leads, even small alignment errors in making the contact will result in incorrect connections. Connections to the microcircuit that are misaligned or otherwise incorrect will cause the test equipment to identify the device under test (DUT) as defective, even though the reason for the failure is the incorrect electrical connection between the test equipment and the DUT rather than defects in the DUT itself.
Test equipment in current use has an array of test contacts that match the pattern of the microcircuit lead array. The array of test contacts is supported in a structure that precisely maintains the alignment of the contacts relative to each other. An alignment template or board aligns the microcircuit itself with the test contacts. The test contacts and the alignment board are mounted on a load board having conductive pads that make electrical connection to the test contacts. The load board pads are connected to circuit paths, also known as traces, that carry the signals and power between the test equipment electronics and the test contacts.
A certain type of common test contact has the form of a thin, elongate lever or arm in a stylized S shape. U.S. Pat. No. 5,967,848 shows test contacts of this type, see FIG. 6 therein for example. In use, a point near one end of each test contact rests on a load board contact. A point near the other end of each test contact rests on a DUT lead. A guide has walls that define a set of side-by-side slots that holds these test contacts in alignment with the DUT leads and the traces. The pitch of the slots is chosen to align the individual slots with the DUT leads.
The levers are held in place within the slots by retaining pins or bars made of a resilient material such as an elastomer. The retaining bars pass through holes in the walls defining the guide's slots to fit inside the curves defined by the S shape of the levers. The '848 patent shows how the elastomeric bars intercept the levers. The resiliency in the retaining bars provides compliance in the positioning of the lever contacts when a presser foot presses an array of DUT leads against the lever contacts. The retaining bars deform in shear slightly to provide consistent force between each contact lever and its associated load board contact and DUT lead.
One problem that arises in microcircuit testing is the potential for electrical resistance between the lead and its test contact. When this resistance is too high, signals that the microcircuit produces during the test may appear to be too low. That DUT may then be rejected as faulty even though the problem is actually a poor contact between a DUT lead and the test contact. This poor contact is strictly an artifact of the testing and is unlikely to be present once the DUT has been soldered onto a circuit board.
“Kelvin” testing refers to a process where the test equipment provides two test contacts, often referred to as “force” and “sense” contacts, for each DUT lead. The force contact carries the signal to or from the DUT for testing DUT operation, and can also be called the signal contact. The sense contact carries the Kelvin signal for assuring that the contact to DUT lead is good, and can also be called the Kelvin contact.
A preliminary part of the test procedure measures the resistance between the two test contacts. If this value is high, one or both of the two test contacts are not making good electrical contact to the microcircuit terminal. If the possibility of high resistance at this interface will affect the accuracy of the actual testing of the microcircuit performance, then the issue can be addressed according to the provisions of the testing protocol. For example, a Kelvin test failure may suggest that the operator should adjust the test equipment or replace the test contact.
Kelvin testing provides additional assurance that each test contact has made good electrical connection with a DUT lead. When the Kelvin test shows good contact at each DUT lead, then it is reasonable to conclude that a failure in the remainder of the test resulted from a defective DUT. Thus, a Kelvin test often eliminates falsely detecting that a DUT is defective.
U.S. Pat. No. 5,967,848 shows lever-type contacts that have on the sides thereof, electrical components such as capacitors, inductors, transistors, and even microcircuits. These components can for example be used to match impedances of a microcircuit lead and the load board circuitry to which it is connected.